Referring to FIG. 1, a conventional transformer 1 includes an E-shaped magnetic core 13, an I-shaped magnetic core 14, and a primary coil 11 and a secondary coil 12 that are wound around and tightly coupled to the E-shaped magnetic core 13. The magnetic cores 13, 14 are generally made of a single material, and thus there must be an air gap between the magnetic cores 13, 14 for preventing magnetic saturation, resulting in negative magnetic damping effect and thus limiting output electrical energy. In addition, a counter electromotive force generated at the output side (OUT) of the conventional transformer 1 may directly impact the input side (IN) of the conventional transformer 1, resulting in great eddy current loss. As a result, even if the conventional transformer 1 has high conversion efficiency, it can only be used for transmission or transfer of electrical energy.